Your pain could be due to muscle tension or spasms, possibly a pinched nerve. You may benefit from an evaluation by a Sports Medicine Physician and a Physical Therapist to sort the problems out. It is possible though you have been riding your narrow bars for years, that your back and shoulders may need to go with a wider set of bars and possibly some bar ends to allow you alternate positions to relieve the stress. Your body may not be quite as limber as when you were a younger rider. I know I had to do some adjustments on my bike as the years have gained on me. You may also benefit from some regular stretching exercises for your back and shoulder muscles. This is where the Physical Therapist evaluation could be of great benefit to see if there are some specific muscle groups that would benefit from some therapies and stretches.